Extractive distillation process



Oct. r14, 1958 P. MUNK 2,856,442

EXTRACTIVE DISTILLATION PROCESS Filed Dec. e, 1954 INVENTOR CII-13d REFORMING 20N E j /2 T-roRNEYs United States Patent EXTRACTIVE DlSTILLATION PROCESS Paul Munk, Berkeley, Calif., assignor to California Research Corporation, San Francisco, Calif., a corporation of Delaware Application December 6, 1954, Serial No. 473,072

2 Claims. (Cl. 260-674) This invention relates to an extractive distillation process for recovering substantially pure aromatic compounds from hydrocarbon distillate fractions containing the desired aromatic compound along with other aromatic as well as nonaromatic constituents, the invention finding particular application in the recovery of benzene from catalytic reformates.

The established specifications for substantially pure (nitration grade) aromatic compounds such as benzene, toluene and xylcne are such as to permit the presence of only trace amounts of other aromatic compounds (as well as of nonaromatic contaminants) than the desired product if the product is to meet the required distillation range specifications. This is borne out by the fact that the presence of as little as 0.1% of the next higher aromatic, in the case of either benzene or toluene, is sufficient to bring the boiling range to the limit of the specification which calls for a distillation rang-e of 1 C., initial to dry. Since the distillate fractions initially obtained following various petroleum refining and other operations which are productive of aromatic products normally contain :substantial amounts of more than one aromatic compound (along with various nonaromatic materials), the practice which is now followed by a large segment of the industry is to redistill such fractions under rigorous conditions whereby a fraction is obtained which contains the desired aromatic compound but is fr-ee of other aromatic contaminants. Thus, in the case of the benzenecontaining fraction recovered from a catalytic reforming operation, which normally contains a substantial amount of toluene along with associated nonaromatic components, the practice is to subject this fraction to one or more preliminary fractional distillation steps designed to provide a benzene heart cut which is substantially free (less than 0.1% by volume in terms of the benzene content) of any toluene, and to thereafter subject this concentrate to an extractive distillation treatment whereby contaminating nonar-omatic components are removed overhead, while the bottoms, which contain the benzene along with the extractive solvent, are then distilled so as to recover substantially pure benzene.

The foregoing method has the disadvantage that a heart cut which contains but a lsingle aromatic compound and is free of contaminating aromatic components can only be obtained by a practice of the most exacting techniques involving the use of high reflux ratios and distillation columns containing as many as 75 to 80 or more trays, thereby greatly adding to the cost of the recovery procedure. To remedy this and other deficiencies of the process described above and to otherwise provide a generally superior method for recovering pure aromatics, there is disclosed in co-pending application Serial No. 448,456 of Robert S. Worthington and Harrison M. Lavender, Jr., tiled August 9, 1954, a method whereby the reformate is distilled under such relatively non-severe conditions as to provide a heart cut which not only contains the desired aromatic component (e. g., benzene) 'ice but also a minor percentage (usually from about 0.5 to 5%, in terms of the weight of the desired aromatic compound)` of a higher boiling aromatic c-ompound present in the reformate, as well as various non-aromatic contaminants. Thus, with benzene, the higher boiling contaminant will normally be toluene. Such a heart cut, which can readily be obtained by the use of simple and relatively inexpensive fractionating equipment is then passed, along with a higher boiling extractive solvent (e. g., phenol), to an extractive `distillation column where the lighter nonaromatic components of the feed are taken overhead while the aromatic components, along with any remaining nonaromatic contaminants, are withdrawn as bottoms. The bott-oms are then passed to a solvent stripping column from which the desired aromatic component is taken overhead in the pure state, while the bott-oms are recycled back to the extractive distillation column.

This method, while constituting a decided step forward in the art, has the disadvantage that the higher boilingA aromatic component (e. g., toluene) present in the heart cut feed to the extractive distillation column builds up in the solvent recycle stream and thus imposes an added burden on the system in addition to reducing the capacity of the unit when solvent recycle facilities or extractive distillation column capacities are limiting factors. While the method of the aforesaid co-pending application in part overcomes this disadvantage by diverting a portion of the bottoms from the stripping unit to a second stripping column where the nonsolvent components are taken overhead and from which the solvent bottoms are returned to the main solvent recycle stream, it has been found that even when as much as 10% of the bottoms are diverted to the second stripper, the solvent recycle stream to the extractive distillation column will still contain approximately 10% by volume of the aromatic component as the system comes to equilibrium. Doubling the amount of bott-oms diverted (i. e., to 20% still leaves approximately 5% aromatics in the recycle stream, while the value is reduced to a level of about 2 to 3% even by passing 40% lof the bottoms from the first to the second stripping column. While this latter amount of contaminant can be tolerated in some instances, it would be desirable if the same could be reduced to the level of 0.5 to 1.5%. Further, in the interests of economy of operation it is desirable to maintain this low aromatics content while restricting the feed to the secondary stripping column to an amount equivalent to no more than about 15% of the volume of the bottoms normally taken out of the first stripping column, and preferably this percentage should be reduced to 10% or less.

The present invention, which has as itsobject, among of the process described in the said oo-pending application, rests on the discovery that said process can be practiced in such a fashion that the aromatic content of the solvent recycle stream to the extractive distillation column is reduced to negligible proportions by employing as the feed to the second stripping column a vapor side cut taken from a lower zone in the first, or primary solvent stripping column. By operating in this fashion it is possible to maintain the concentration of the aromatic contaminant (e. g., toluene) in the solvent recycle stream to the extractive distillation unit at a level below about 1.5% by volume, while still limiting the feed to the second stripping column to a relative amount not exceeding about 15% of that withdrawn as bottoms from the first stripping column.

While the process has been described above in its the preparation of individual pure aromatic compounds,

such as benzene, .from catalytic reformate fractions, particularly Platformates derived by the catalytic reforming of naphtha fractions over a platinum-alumina reforming catalyst. In this process, as it applies to the production of benzene, a petroleum naphtha fraction consisting essentially lof a mixture of naphthe-nic, paraiiini'c and aromatic lhydrocarbons having from six to seven carbon atoms Vin the molecule, after lbeing contacted in the vapor phase with a platinum-alumina reforming catalyst under suitable conversion conditions to largely convert the naphthen'ic hydrocarbons containing six carbon atoms to benzene and the seven carbon atom naphthenic hydrocarbons to toluene, -a'nd to largely isomerize the straight carbon :chain paraffnic hydrocarbons to ybranched chai-n compounds, is fractionally distilled in one lor more columns to produce a ben-zene heart cut containing substantially all of the benzene Iof the 'conversion product, along with -no'naromatic components and 'from labout L'0.2 to 5% by volume lof toluene, based on .the volume of benzene -in 'the concentrate. Following the preparation of the concentrate, the latter is extractively distilled in the presence of ay suitable solvent (e. g., phenol) under such conditions that the relatively lower boiling nonaromatic components are separated as an overhead product, while all of the benzene and toluene, along with the higher boiling nonaromatic portion of the concentrate, is Withdrawn as bottoms. Thereafter, the bottoms are passed -to a benzene stripping column at an elevated temperature adapted to distill benzene of the desired purity overhead without vaporizing the other components of the feed, while a vapor 'side cut rich in -toluene is taken between about vthe first and -eighth trays Aand 'passed to a second, or toluene lstripping column, the bottoms from said benzene stripping column, containing less 'than about 1.5% by volume of toluene, being recycled tothe extractive distillation column for admixture with the ncoming benzene concentrate. From the toluene stripping column, a stream made up predominantly of toluene, but also containing minor percentages of benzene and C7 naphthene's, is taken overhead and returned to the benzene heart cut still (identiiied as a re-run column in the drawing) while the bottoms are combined with those obtained from the benzene stripping column.

While 'phenol has been given above as an example of an extractive solvent, and is, in fact, the preferred solvent for use in this invention 'when recovering benzene or toluene, -a wide 'variety of other solvents can be employed. In general, the solvent selected should have a boiling point which is Well above the end point of the stock being treated and which can thus be readily separated from the dissolved components. Thus, suitable solvents which can be employed in a practice of this invention vare: phenol, aniline, cresylic acids, alkyl ph'enols, various polyglycols including di, triand tetraethylene glycol, carbitols, diethanolamine, nitrobenzene, quinoline, furfural alcohol, furfural, monoglycerol ethers, resorcinol,

xylidine, acetone and acetone-water mixtures.

The process of the present invention can be illustrated in one embodiment thereof by reference to the appended drawing wherein the ligure represents a iiow dia-gram of a process 'for the catalytic reforming of a hydrocarbon feed, and particularly for the recovery of pure benzene from the 'resulting reformate.

In the drawing, a lfeed mixture comprising a petroleum naphtha fraction consisting primarily of a mixture -off naphthenic, paraini'c and aromatic hydrocarbons having from 6to 7'carbon atoms 'in the molecule is passed in the vapor phase through line '10 into reforming zone `11 wherein the feed comes into contact with a reforming catalyst under elevated "conditions of temperature and pressure suitable for effecting the desired conversion of C8 naphthenes to benzene and the C7 naphthencs to toluene, and to largely isomerize the straight-chain paranc hydrocarbons present to branched-chain compounds. From the reforming zone the resulting reformate passes through line 12 into a stabilizer column 13 provided with a suitable reboiler 14, from which column the lighter fractions comprising predominantly C3-C4 hydrocarbons are taken overhead through line 15 and condenser 16, with a portion of said lighter hydrocarbons being sent to product recovery through line 17, while the balance is returned as reflux to the column through line 18.

The `bottoms from stabilizer column 13 are Withdrawn through line 19 'and Apassed into a top ping still 20 provided with a reboiler 21. From this still a lighter vfraction comprising essentially C5 and C6 hydrocarbons boiling below hexane is taken overhead through line 22 and condenser 23, with a portion of the condensate being sent to product recovery through line 24, while the balance is returned to the column as reflux through line 25.

The bottoms from the topping still 20 are withdrawn through line 26 Vand `charged into a mid-portion of a rerun column 30 lprovided with 'a reboiler 31. From this column Aa benzene heart cut fraction is 'taken overhead` throughline 32 and 'condenser.33, which fraction contains :substantially all of 'the benzene supplied to the column, as well as toluene in an amount equal to from about 0.2 to 5% by volume, based on the benzene content of the fraction, along with a substantial proportion of nonaromatic components. Of the resulting condensate, a portion is returned to the column as re'ux through line 34, while the balance of the stream is passed through line 35 vinto a mid-point of an extractive distillation column 36 'having a reboiler 38. The balance ofthe toluene introduced into re-r'un column 30 'through line 26 is removed from said column through line 37, 'along with no naromatic hydrocarbons boiling above benzene, and is sent to a'suitable toluene recovery system (not shown).

`In a representative run, the benzene concentrate in line 35 contains 28.6% benzene, 1.4% toluene and 70% nonaro'matics, and is supplied to the extractive distillation column 36 ata rate of 1000 volumes per day, with phenol being introduced to the column through line 50 at a rate of 1400 volumes per day. The temperature in column 36 is so regulated that the hydrocarbons boiling below benzene are taken overhead through line 39 and condenser 40 at `arate of 3800 volumes per day, with 3100 volumes per day being returned to the column as reflux through line 42, while a raiiina'te :stream amounting to 700 volumes per day is sent t-o a suitable recovery system (not shown) 'through line 41, said rainate (at 'a still Vhead temperature 'of 178 F. and under the described reflux conditions) Aconsisting of 96.4% nonaromatics and but 3.6% benzene.

The vbottoms from the column 36, which 'contain the desired benzene along with associated higher boiling aromatic (toluene) and nonaromatic contaminants, as well as the solvent, are supplied to a mid-point :of a benzene stripp'er columnv 44 provided with a reboiler 45. From the column 44 (when operated, for example, at a still head temperature of 183 F. and at reflux ratio 'of about 6 to 1) a benzene stream consisting of 99.2% benzene and 0.48% nonaromatics is 'taken overhead through line 46 'and condenser 47 'at a rate of 1680 volumesper day, of which 280 volumes is discharged through line 48, while the balance is returned to the column as reiiux through 'line 49. The stream lin line 48 may, if desired, be passed to a treating zone Where the benzene is subjected to the conventionalv nishing treatments required for 'the production of a product of nitration grade.

The major portion ofthe phenol solvent introduced vinto column 44 is withdrawn as bottoms through line 50 at a temperature of about 390 F. 'and recycled to the extractive distillation column 36. The balance of the solvent is withdrawn'through line 51 as a vapor side cut rich in toluene, this line being positioned between the fifth and sixth trays of column 44. Specifically, 1280 volumes per day of bottoms, at a temperature of 390 F. and containing approximately 1% by volume of toluene, are withdrawn through line 50, While the side cut in line 51, taken at about 375 F. totals 13S volumes per day and is made up of 120 volumes phenol, 12 volumes toluene, and 3 volumes benzene with nonaromatic components representing the balance. The product in line 51, preferably after being cooled to a temperature of about 360 F., is passed to a mid-point of toluene stripping column 52 provided with a reboiler 53. From this column an overhead stream at a temperature of 235 F. is taken through line 54 and condenser S5 at a rate of approximately 54 volumes per day of which 36 volumes is returned to the column as reilux through line 56, while the balance is withdrawn through line 57. The bottoms from column 52, consisting essentially of phenol, are withdrawn through line 58 at a temperature of about 390 F. and at a rate of 120 volumes per day, this stream being joined with that in line 50 for recycle to column 36.

However, the overhead product withdrawn through line 57 at a rate of 18 volumes per day (made up of 12 volumes toluene, 3 volumes benzene, and the balance consisting predominantly of naphthenic constituents) is returned to column 30 via line 26. In this fashion, the toluene and benzene components of the overhead stream from the toluene stripping column 52 are retained in the system for eventual recovery through the benzene and toluene oitake lines 48 and 37, respectively.

While in the foregoing description the vapor side cut line 51 from the benzene stripping column has been positioned between the fth and sixth trays of that column, the position of this line may be varied somewhat, as from about the first to the eighth tray, preferably from the fourth to the eighth tray, depending on the conditions encountered within a particular unit. Thus, it is desirable to obtain a vaporous side cut fraction which is relatively low in the solvent component as well as in that taken overhead from the column, yet which is as rich as possible in the aromatic contaminant, i. e., toluene in the case of the unit described above. Thus, if the line is positioned higher than about the seventh or eighth trays in the co1- umn, the side cut obtained becomes unduly rich in the overhead component (i. e., benzene), while if the line is positioned below about the fourth tray, the product recovered in line 51 becomes unduly rich in the phenol or other solvent component employed and thus tends to overload the second stripping column 52, although this may be preferable to increasing the recycle of the overhead component, here benzene.

In the foregoing description, the various percentages given are on a volume basis. Further, in the ow diagram various of the required pumps, heat exchangers, valves and other items of ilow control equipment have been omitted in the interests of simplicity and clarity of expression, the placement of such auxiliary equipment being evident to those skilled in the art.

While the character of this invention has been described in detail and numerous illustrative examples given, this had been done by way of illustration only and with the intention that no limitation should be imposed upon the invention thereby. It will be apparent to those skilled in the art that numerous modifications and variations may be effected in the practice of this invention which is of the scope of the claims appended hereto.

I claim:

1. In a process for recovering substantially pure benzene from a catalytic reformate containing benzene, toluene and nonaromatic components, the steps comprising freeing the reformate of lighter nonaromatic hydrocarbon constituents; passing the resulting bottoms product so obtained to a re-run column from which a concentrate containing substantially all the benzene content of said reformate, along with nonaromatic components and from about 0.2 to 5% by volume of toluene, is taken overhead, and from which a fraction comprised in major portion of toluene is withdrawn as bottoms; passing said concentrate to an eXtractive distillation column, and there contacting the concentrate with a selective solvent at elevated temperatures whereby the relatively low boiling nonaromatic components are separated as an overhead product, while withdrawing a bottoms product containing substantially all the benzene and toluene content of the concentrate, along with solvent; passing said bottoms product to a benzene stripping column maintained at a temperature adapted to distill the benzene overhead while withdrawing as botto-ms a solvent fraction low in toluene, and, from a lower zone of `said stripping column, a vaporous side cut relatively rich in toluene; passing said side cut to a toluene stripping column maintained at a temperature adapted to distill overhead the toluene and other nonsolvent components present, while withdrawing as bottoms a solvent fraction; recycling said toluene-containing overhead fraction to the rerun column; and recycling the bottoms from said stripping columns to the extractive distillation column.

2. The process of claim 1 wherein the selective solvent employed in the extractive distillation column is phenol.

References Cited in the tile of this patent UNITED STATES PATENTS 2,325,379 Durrum .Tuly 27, 1943 2,366,570 Souders et al. Jan. 2, 1945 2,707,716 Price May 3, 1955 2,731,506 Love et al. Oct. 17, 1956 OTHER REFERENCES Dunn et al.: Oil and Gas Journal, vol. 51, issue No. 5 (June 9, 1952), pages 68, 69, 70 and 96. 

1. IN A PROCESS FOR RECOVERING SUBSTANTIALLY PURE BENZENE FROM A CATALYTIC REFORMATE CONTAINING BENZENE, TOLUENE AND NONAROMATIC COMPONENTS, THE STEPS COMPRISING FREEING THE REFORMATE OF LIGHTER NONAROMATIC HYDROCARBON CONSTITUENTS; PASSING THE RESULTING BOTTOMS PRODUCT SO OBTAINED TO A RE-RUN COLUMN FROM WHICH A CONCENTRATE CONTAINING SUBSTANTIALLY ALL THE BENZENE CONTENT OF SAID REFORMATE, ALONG WITH NONAROMATIC COMPONENTS AND FROM ABOUT 0.2 TO 5% BY VOLUME OF TOLUENE, IS TAKEN OVERHEAD AND FROM WHICH A FRACTION COMPRISED IN MAJOR PORTION OF TOLUENE IS WITHDRAWN AS BOTTOMS; PASSING SAID CONCENTRATE TO AN EXTRATIVE DISTILLATION COLUMN, AND THERE CONTACTING THE CONCENTRATE WITH A SELECTIVE SOLVENT AT ELEVATED TEMPERATURES WHEREBY THE RELATIVELY LOW BOILING NONAROMATIC COMPONENTS ARE SEPARATED AS AN OVERHEAD PRODUCT, WHILE WITHDRAWING A BOTTOMS PRODUCT CONTAIN- 